There are some absolutely WONDERFUL resources!

The best of the best is A Guide to Irish Law by Fergus Kelly.

The Dublin Institute for Advanced Studies has done a series on early Irish Law which includes the above listed title as well as Bechbretha, an Old Irish Law tract on bee-keeping edited by Mr. Kelly and Thomas Charles Edwards, Uraicecht na Riar, The Poetic Grades in Early Irish Law edited by Liam Breatnach.

The Hywel Dda has been published as well. The copy I have was edited by Dafydd Jenkins.

The Brehon Laws were written down first, from the 5th - 8th centuries, whereas the Law of Hywell Dda was first written down much later and already shows outside influences which must be stripped away to find the Celtic root of the laws.

What these books show us is that the insular celtic (at least) society was far more complicated and stratified than is commonly believed.

One basic of Irish law was rank. Simply said, the higher your rank, the more legal standing you'd have. "An offence against a person of higher rank entails a greater penalty than the same offence against that of a person of lower rank. Similarily, the oath of a person of higher rank automatically outweighs that of a person of lower rank." (Kelly, 1998 p. 7)